The present invention is related to thermoplastic elastomeric compositions particularly useful for tire and other industrial rubber applications, reinforced or otherwise, that require impermeability characteristics.
EP 0 722 850 B1 discloses a low-permeability thermoplastic elastomeric composition that is excellent as an innerliner in pneumatic tires. This composition comprises a low permeability thermoplastic dispersed in a low permeability rubber. EP 0 969 039 A1 discloses a similar composition and teaches that the small particle size rubber dispersed in the thermoplastic was important to achieve acceptable durability of the resulting composition. The goal of the thermoplastic elastomeric material in both references is a highly impermeable material that has the processability of a plastic and the high flexibility of the elastomer.
To obtain the desired characteristics and properties of a thermoplastic elastomer, particularly useful in vulcanized or cured products, the elastomeric component of the thermoplastic elastomer is cured to at least a fifty percent cure state, most preferably a 100 percent cure state. Curing of the elastomer in the thermoplastic elastomer is generally accomplished by the incorporation of a combination of curing agents and accelerators, with the overall mixture of such components referred to as the cure system or cure package. Because only curing of the elastomer is desired, the curing agents and accelerators used are based upon what is conventionally used to cure the elastomer absent the presence of the thermoplastic resin; i.e., the curatives used when compounding only an elastomeric compound.
For isoolefin based elastomers, such as C4 to C7 isoolefins like isobutylene copolymer, conventional cure systems include a primary cure agent such as sulfur, organometallic compounds, phenolic resins, radical initiators, quinine dioximes, peroxides, and metal oxides. Because the use of these primary curing agents alone require a long cure time at conventional elastomeric curing temperatures, generally less than 200° C., the curatives are conventionally used in conjunction with accelerators.
Conventional accelerators include metal stearate complexes (e.g., the stearate salts of Zn, Ca, Mg, and Al), alone or in combination with stearic acid or other organic acids and either a sulfur compound or an alkyl or aryl peroxide compound or diazo free radical initiators and accelerators. Other accelerators, known to those in the elastomeric compound art, include amines, guanidines, thioureas, thiazoles, thiurams, sulfenamides, sulfenimides, thiocarbamates, xanthates, Lewis acids, and the like.
An analysis of the torque properties of a conventional elastomer versus time shows a slow increase in the torque, S′ (the elastic stiffness measured by a moving die rheometer). When such an elastomer is used in a thermoplastic elastomer, this behavior can be undesirable as it indicates that the thermoplastic elastomer may be prone to undergo additional chemical reactions, including reversion or degradation of the cure, during higher temperature article formation operations, such as film blowing or casting of the thermoplastic elastomer at temperatures greater than 220° C.
The need exists to determine a cure system for the thermoplastic elastomers that achieve sufficient cure of the elastomer during formation of the thermoplastic elastomer at high temperatures that also provides for a stable cure thermoplastic elastomer during later high temperature article forming operations.